1. Field of the Invention
The present invention relates to a digital-to-analog (D/A) converter.
2. Description of the Prior Art
In recent years, there has been a sudden increase in the demand for compact disc players, accompanied by the earnest desire for improvement in the tone quality. It is the conversion characteristics of the D/A converter as used in such apparatus that largely affects the tone quality, and thus various improvements in it have been proposed. One of the improvements is that which utilizes dithers.
The improvement aims at averaging the converting steps by means of dithers. That is, in accordance with such proposed improvement, random data corresponding to dithers is added to digital data when the latter is converted into an analog form, and the random data is subtracted after the D/A conversion. FIG. 5 shows a typical circuit embodying such an improvement, in which digital data, which is shown in FIG. 6A and is obtained by sampling at a frequency f.sub.s, is added by an adder CA to random data (shown in FIG. 6B) of a frequency f.sub.s from a random data generating circuit RD. A selector alternately S selects the outputs from the adder CA and from the random data generating circuit alternately as shown in FIG. 6D, and a D/A converter DT, which receives the output from the selector S, generates an analog signal which corresponds to the input data, as shown in FIG. 7E, the analog signal portions of which correspond to the random data being sampled by a sample and hold circuit SH (as shown in FIG. 7F). A subtractor RE generates an output which corresponds to the difference between such sampled analog signal portions and the analog signal from the D/A converter DT. Specifically, the analog signal portions corresponding to the random data are subtracted from the output from the D/A converter DT, whereby an analog signal corresponding to the digital data is generated as shown in FIG. 7G. A deglitcher DG extracts stable portions of this analog signal as shown in FIG. 7H, excluding unstable portions at transition points of the analog signal. The thus obtained signal is passed through suitable circuit means such as a low-pass filter (not shown), whereby it is reproduced as an acoustic signal.
In the above-described prior art circuit, the output analog signal from the subtractor RE contains a resistance thermal noise power component which is twice as large as that which is contained in the input to the subtractor RE, the circuit arrangement therefore has a relatively poor signal-to-noise ratio.
Specifically, the subtraction will double, in terms of electric power, the resistance thermal noise component contained in the output from the subtractor RE, since the resistance thermal noise contained in the output from the D/A converter DT and the resistance thermal noise contained in the output from the sample hold circuit SH are not correlated together. This will deteriorate the signal-to-noise ratio, thereby degrading the tone quality.
An object of the present invention is therefore to improve the above-mentioned shortcomings in order to enhance the signal-to-noise ratio.